The epitope specificities of monoclonal antibodies against carcinoembryonic antigen (CEA) were determined in a solid-phase, biotin-avidin enzyme immunoassay. Constant amounts of biotin-labeled antibodies were incubated with the immobilized antigen in the presence of decreasing amounts of unlabeled antibodies. The biotinylated antibodies bound to the antigen were then reacted with avidin-peroxidase conjugates. The activity of the bound peroxidase was determined by the use of o-phenylenediamine and hydrogen peroxide.